1. Field of the Invention
This invention relates to a liquid explosive which is particularly suitable for fracturing a geological formation adjacent a well bore, for bringing in the well or for increasing the productivity of a well which has substantially ceased to produce oil, water or gas. It is also suitable for other applications, such as quarrying, especially where an explosive composition is required which will conform to the formation in which it is placed and will permit propagation of the explosion through the fine fissures often encountered in geological fracturing.
To bring in a well, after it has been drilled, it is usually necessary to increase the permeability of the producing formation to stimulate flow in the well. This was commonly done by "shooting" the well with a nitroglycerin charge, acidizing (in certain types of formation) or hydraulic fracturing. When a formerly productive well has ceased to produce, the pay zone is similarly fractured to reactivate the well. The purpose of fracturing is to increase the permeability of the productive formation or pay zone, permitting flow from the producing formation into and up the well bore.
Explosive fracturing was originally carried out by placing a nitroglycerin charge in the well bore and detonating it. Such charge could be desensitized nitroglycerin in liquid or gel form or a mixture of pure nitroglycerin and ethylene glycol dinitrate (EGDN). The disadvantages of nitroglycerin, used for many years for this purpose, are many. For example, it is extremely shock sensitive and difficult to handle and transport. All nitroglycerins are too sensitive, for example, to be pumped or poured into a well and must be carefully placed there, either in gel form or as desensitized liquid. Liquid and slurry explosives other than nitroglycerin have been tried but, in general, have not been successful for reasons including instability, segregation of constituents, detonation problems and vulnerability to leaching and dilution by fluids in well bores.
"Bore shots", as fracturing operations are called when the explosive is placed wholly within the well bore, tend to destroy the bore, and while they do result in exposing more of the surface of the formation to the bore, they do not materially increase the permeability of the formation itself. It has been found that the fracturing of a large volume of the formation, for optimum increase in permeability, requires an explosive having a small critical diameter which can be loaded back into the formation itself, permitting it to propagate an explosion through the narrow fissures and crevices of the formation. The term "narrow fissures" or "fine fissures" as used herein means those narrow cracks or openings found in geological strata adjacent well bores, commonly formed by hydraulic fracturing and having widths from approximately one-quarter inch down to a fraction of a millimeter. Nitroglycerin, for example, has been shown to propagate an explosion in fissures as narrow as 1/32 inch, according to Eakin and Miller, "Explosives Research to Improve Flow Through Low Permeability Rock", Paper No. SPE 1715 (Third SPE of AIME Drilling and Rock Mechanics Conference, 1967). Other methods limit effective propagation of the explosion to a relatively small radius around the well bore bottom.
2. History of the Prior Art
To overcome these drawbacks, experiments have been conducted for several decades with liquid explosives other than nitroglycerin, including slurry explosives, which are dispersions of solid explosives or of one or more explosive constituents suspended in water, oil, or some other medium. Liquid (including slurry) explosives have the advantage of being able to conform to, and thus more readily fill, the well bore, resulting in greater explosive power. It is important that explosives of this kind be capable of being pressured back into the geological formation adjacent the well bore in order to obtain complete, even, and adequate fracturing of the formation and to minimize damage to the well bore and to any casing installed in the well.
A serious problem in liquid and slurry explosives developed to date has been their inability to undergo pressurization into a well formation, and still be capable of consistent and reliable detonation without the necessity of using complex and expensive detonating systems. In certain instances, indispensible constituents of the explosive are filtered out in passing through the narrow fissures and pores of the formation. In other cases, exposure to fluids in the well bore or formation may cause dilution of the explosive, rendering it incapable of detonation, or may leach out certain of its essential constituents.
Other explosive compositions are highly diameter sensitive, meaning that they are incapable of being detonated in cross-sections of less than a certain diameter. Diameter sensitivity is a measure of the capability of an explosive compound to propagate an explosion in narrow passages, such as geological fissures. Diameter sensitivity as used herein has reference to the ability of a composition to propagate an explosion along a tube filled with the composition, containing a restricted orifice of a given diameter, so that the explosion propagates past the orifice and is not extinguished by the reduced diameter of the composition. Thus, an explosive with a diameter sensitivity (or critical diameter) of 1 inch, placed in a tube of greater diameter, will propagate an explosion past a 1 inch diameter orifice, but is incapable of propagating an explosion past an orifice of lesser diameter. This indicates that the same explosive will propagate an explosion in a 1 inch diameter geological fissure.
Explosive compositions are known having very low critical diameters; for example, nitroparaffin-base compositions disclosed in U.S. Pat. No. 3,663,324 have critical diameters of the order of 1/64 inch. However, those compositions contain finely divided aluminum, which may react with certain acids found in some geological environments and, in addition, produces too brisant an explosive for some applications. A highly brisant explosive tends to rubblize the formation rather than simply fracturing it, creating large amounts of debris which clog the formation and impede the flow of oil or gas from it.
Generally, in an explosive, the desirable property of lower critical diameter goes hand in hand with the undesirable property, for formation fracturing, of higher brisance. A concomitant drawback of explosives with low critical diameters is ofttimes a higher impact sensitivity than is compatible with the very stringent safety requirements of explosive fracturing.
It is known that the addition to an explosive of materials of a different sonic velocity from that of the explosive medium causes the shock wave of the explosion to generate heat at the boundary of the additive, tending to reinforce and sustain or enhance propagation of the explosion. The added material may be a liquid, gaseous or solid phase. See, for example, U.S. Pat. No. 3,456,589, issued July 22, 1969, to Dow Chemical Company, which discloses explosives containing air-filled glass microspheres as a sensitizing agent. However, the effects of such sensitization on critical diameter, brisance and impact sensitivity are difficult to predict and are often incompatible with the requirements of formation fracturing.